1. Field of the Invention
The present invention relates to an electrophotographic photosensitive member, and a process cartridge and an electrophotographic apparatus each including an electrophotographic photosensitive member.
2. Description of the Related Art
In recent years, electrophotographic photosensitive members that use an organic photoconductive material (charge generation material) have been used as electrophotographic photosensitive members included in process cartridges and electrophotographic apparatuses. Electrophotographic photosensitive members generally include a support and a photosensitive layer formed on the support.
Furthermore, a conductive layer containing conductive particles (metal oxide particles) is disposed between the support and the photosensitive layer for the purpose of covering surface defects of the support and protecting the photosensitive layer from electrical breakdown. However, the potential of the conductive layer containing metal oxide particles easily varies due to environmental changes in temperature and humidity. There is a technique of improving the potential characteristics by improving metal oxide particles. Japanese Unexamined Patent Application Publication No. 2004-151349 describes a technique of using tantalum-doped tin oxide particles in a conductive layer. Japanese Unexamined Patent Application Publication No. 2007-187771 describes a technique of incorporating two types of metal oxide particles having different average particle sizes in an undercoat layer (conductive layer). Japanese Unexamined Patent Application Publication No. 4-191861 describes a technique of incorporating a zinc oxide powder and a tin oxide powder in an undercoat layer.
In recent years, the opportunity to output a large number of identical images within a short time has increased with the realization of high-speed electrophotographic apparatuses. However, in this case, an image defect called a pattern memory is easily caused.
The term “pattern memory” refers to a phenomenon in which, when a solid black image or a halftone image is output after a large number of images including vertical lines 306 (lines extending in a direction in which a recording medium moves), such as an image 301 illustrated in FIG. 4, are continuously output, memories are generated in portions where the vertical lines were formed. Specifically, when a solid black image 302 is output after a large number of images 301 including vertical lines 306 in FIG. 4 are continuously output, an image output as a solid black image is an image 304 including vertical lines 307 formed by the hysteresis of the vertical lines 306 in FIG. 4. Furthermore, when a halftone image 303 is output after a large number of images 301 in FIG. 4 are continuously output, an image output as a halftone image is an image 305 including vertical lines 308 formed by the hysteresis of the vertical lines 306 in FIG. 4 as in the case of the solid black image.
As a result of studies conducted by the present inventors, it has been found that such a pattern memory is sometimes caused in the electrophotographic photosensitive members including a conductive layer and described in the above documents, and thus there is a room for further improvement.
In the conductive layer containing metal oxide particles, an increase in residual potential and the formation of cracks in a conductive layer are in a trade-off relationship. Therefore, the suppression of an increase in residual potential and formation of cracks is required in addition to the suppression of formation of the pattern memory.
The present invention provides an electrophotographic photosensitive member in which an increase in residual potential, formation of a pattern memory, and formation of cracks in a conductive layer are suppressed, and a process cartridge and an electrophotographic apparatus including the electrophotographic photosensitive member.